CHAPTER XIV.

RECAPITULATION AND CONCLUSION.

Recapitulation of the difficulties on the theory of Natural Selection.

Recapitulation of the general and special circumstances in its favour.

Causes of the general belief in the immutability of species. How far

the theory of natural selection may be extended. Effects of its

adoption on the study of Natural history. Concluding remarks.

As this whole volume is one long argument, it may be convenient to the

reader to have the leading facts and inferences briefly recapitulated.

That many and grave objections may be advanced against the theory of

descent with modification through natural selection, I do not deny. I

have endeavoured to give to them their full force. Nothing at first can

appear more difficult to believe than that the more complex organs and

instincts should have been perfected, not by means superior to, though

analogous with, human reason, but by the accumulation of innumerable

slight variations, each good for the individual possessor.

Nevertheless, this difficulty, though appearing to our imagination

insuperably great, cannot be considered real if we admit the following

propositions, namely,—that gradations in the perfection of any organ or

instinct, which we may consider, either do now exist or could have

existed, each good of its kind,—that all organs and instincts are, in

ever so slight a degree, variable,—and, lastly, that there is a

struggle for existence leading to the preservation of each profitable

deviation of structure or instinct. The truth of these propositions

cannot, I think, be disputed.

It is, no doubt, extremely difficult even to conjecture by what

gradations many structures have been perfected, more especially amongst

broken and failing groups of organic beings; but we see so many strange

gradations in nature, as is proclaimed by the canon, “Natura non facit

saltum,” that we ought to be extremely cautious in saying that any

organ or instinct, or any whole being, could not have arrived at its

present state by many graduated steps. There are, it must be admitted,

cases of special difficulty on the theory of natural selection; and one

of the most curious of these is the existence of two or three defined

castes of workers or sterile females in the same community of ants; but

I have attempted to show how this difficulty can be mastered.

With respect to the almost universal sterility of species when first

crossed, which forms so remarkable a contrast with the almost universal

fertility of varieties when crossed, I must refer the reader to the

recapitulation of the facts given at the end of the eighth chapter,

which seem to me conclusively to show that this sterility is no more a

special endowment than is the incapacity of two trees to be grafted

together, but that it is incidental on constitutional differences in

the reproductive systems of the intercrossed species. We see the truth

of this conclusion in the vast difference in the result, when the same

two species are crossed reciprocally; that is, when one species is

first used as the father and then as the mother.

The fertility of varieties when intercrossed and of their mongrel

offspring cannot be considered as universal; nor is their very general

fertility surprising when we remember that it is not likely that either

their constitutions or their reproductive systems should have been

profoundly modified. Moreover, most of the

varieties which have been experimentised on have been produced under

domestication; and as domestication apparently tends to eliminate

sterility, we ought not to expect it also to produce sterility.

The sterility of hybrids is a very different case from that of first

crosses, for their reproductive organs are more or less functionally

impotent; whereas in first crosses the organs on both sides are in a

perfect condition. As we continually see that organisms of all kinds

are rendered in some degree sterile from their constitutions having

been disturbed by slightly different and new conditions of life, we

need not feel surprise at hybrids being in some degree sterile, for

their constitutions can hardly fail to have been disturbed from being

compounded of two distinct organisations. This parallelism is supported

by another parallel, but directly opposite, class of facts; namely,

that the vigour and fertility of all organic beings are increased by

slight changes in their conditions of life, and that the offspring of

slightly modified forms or varieties acquire from being crossed

increased vigour and fertility. So that, on the one hand, considerable

changes in the conditions of life and crosses between greatly modified

forms, lessen fertility; and on the other hand, lesser changes in the

conditions of life and crosses between less modified forms, increase

fertility.

Turning to geographical distribution, the difficulties encountered on

the theory of descent with modification are grave enough. All the

individuals of the same species, and all the species of the same genus,

or even higher group, must have descended from common parents; and

therefore, in however distant and isolated parts of the world they are

now found, they must in the course of successive generations have

passed from some one part to the others. We are often wholly unable

even to conjecture how this could have been effected. Yet, as we have

reason to believe that some species have retained the same specific

form for very long periods, enormously long as measured by years, too

much stress ought not to be laid on the occasional wide diffusion of

the same species; for during very long periods of time there will

always be a good chance for wide migration by many means. A broken or

interrupted range may often be accounted for by the extinction of the

species in the intermediate regions. It cannot be denied that we are as

yet very ignorant of the full extent of the various climatal and

geographical changes which have affected the earth during modern

periods; and such changes will obviously have greatly facilitated

migration. As an example, I have attempted to show how potent has been

the influence of the Glacial period on the distribution both of the

same and of representative species throughout the world. We are as yet

profoundly ignorant of the many occasional means of transport. With

respect to distinct species of the same genus inhabiting very distant

and isolated regions, as the process of modification has necessarily

been slow, all the means of migration will have been possible during a

very long period; and consequently the difficulty of the wide diffusion

of species of the same genus is in some degree lessened.

As on the theory of natural selection an interminable number of

intermediate forms must have existed, linking together all the species

in each group by gradations as fine as our present varieties, it may be

asked, Why do we not see these linking forms all around us? Why are not

all organic beings blended together in an inextricable chaos? With

respect to existing forms, we should remember that we have no right to

expect (excepting in rare cases) to discover _directly_ connecting

links between them, but only between each and some extinct and

supplanted form. Even on a wide area, which has during a long period

remained continuous, and of which the climate and other conditions of

life change insensibly in going from a district occupied by one species

into another district occupied by a closely allied species, we have no

just right to expect often to find intermediate varieties in the

intermediate zone. For we have reason to believe that only a few

species are undergoing change at any one period; and all changes are

slowly effected. I have also shown that the intermediate varieties

which will at first probably exist in the intermediate zones, will be

liable to be supplanted by the allied forms on either hand; and the

latter, from existing in greater numbers, will generally be modified

and improved at a quicker rate than the intermediate varieties, which

exist in lesser numbers; so that the intermediate varieties will, in

the long run, be supplanted and exterminated.

On this doctrine of the extermination of an infinitude of connecting

links, between the living and extinct inhabitants of the world, and at

each successive period between the extinct and still older species, why

is not every geological formation charged with such links? Why does not

every collection of fossil remains afford plain evidence of the

gradation and mutation of the forms of life? We meet with no such

evidence, and this is the most obvious and forcible of the many

objections which may be urged against my theory. Why, again, do whole

groups of allied species appear, though certainly they often falsely

appear, to have come in suddenly on the several geological stages? Why

do we not find great piles of strata beneath the Silurian system,

stored with the remains of the progenitors of the Silurian groups of

fossils? For certainly on my theory such

strata must somewhere have been deposited at these ancient and utterly

unknown epochs in the world’s history.

I can answer these questions and grave objections only on the

supposition that the geological record is far more imperfect than most

geologists believe. It cannot be objected that there has not been time

sufficient for any amount of organic change; for the lapse of time has

been so great as to be utterly inappreciable by the human intellect.

The number of specimens in all our museums is absolutely as nothing

compared with the countless generations of countless species which

certainly have existed. We should not be able to recognise a species as

the parent of any one or more species if we were to examine them ever

so closely, unless we likewise possessed many of the intermediate links

between their past or parent and present states; and these many links

we could hardly ever expect to discover, owing to the imperfection of

the geological record. Numerous existing doubtful forms could be named

which are probably varieties; but who will pretend that in future ages

so many fossil links will be discovered, that naturalists will be able

to decide, on the common view, whether or not these doubtful forms are

varieties? As long as most of the links between any two species are

unknown, if any one link or intermediate variety be discovered, it will

simply be classed as another and distinct species. Only a small portion

of the world has been geologically explored. Only organic beings of

certain classes can be preserved in a fossil condition, at least in any

great number. Widely ranging species vary most, and varieties are often

at first local,—both causes rendering the discovery of intermediate

links less likely. Local varieties will not spread into other and

distant regions until they are considerably modified and improved;

and when they do spread, if discovered in a geological formation, they

will appear as if suddenly created there, and will be simply classed as

new species. Most formations have been intermittent in their

accumulation; and their duration, I am inclined to believe, has been

shorter than the average duration of specific forms. Successive

formations are separated from each other by enormous blank intervals of

time; for fossiliferous formations, thick enough to resist future

degradation, can be accumulated only where much sediment is deposited

on the subsiding bed of the sea. During the alternate periods of

elevation and of stationary level the record will be blank. During

these latter periods there will probably be more variability in the

forms of life; during periods of subsidence, more extinction.

With respect to the absence of fossiliferous formations beneath the

lowest Silurian strata, I can only recur to the hypothesis given in the

ninth chapter. That the geological record is imperfect all will admit;

but that it is imperfect to the degree which I require, few will be

inclined to admit. If we look to long enough intervals of time, geology

plainly declares that all species have changed; and they have changed

in the manner which my theory requires, for they have changed slowly

and in a graduated manner. We clearly see this in the fossil remains

from consecutive formations invariably being much more closely related

to each other, than are the fossils from formations distant from each

other in time.

Such is the sum of the several chief objections and difficulties which

may justly be urged against my theory; and I have now briefly

recapitulated the answers and explanations which can be given to them.

I have felt these difficulties far too heavily during many years to

doubt their weight. But it deserves especial notice that the more

important objections relate to questions on which we are confessedly

ignorant; nor do we know how ignorant we are. We do not know all the

possible transitional gradations between the simplest and the most

perfect organs; it cannot be pretended that we know all the varied

means of Distribution during the long lapse of years, or that we know

how imperfect the Geological Record is. Grave as these several

difficulties are, in my judgment they do not overthrow the theory of

descent with modification.

Now let us turn to the other side of the argument. Under domestication

we see much variability. This seems to be mainly due to the

reproductive system being eminently susceptible to changes in the

conditions of life; so that this system, when not rendered impotent,

fails to reproduce offspring exactly like the parent-form. Variability

is governed by many complex laws,—by correlation of growth, by use and

disuse, and by the direct action of the physical conditions of life.

There is much difficulty in ascertaining how much modification our

domestic productions have undergone; but we may safely infer that the

amount has been large, and that modifications can be inherited for long

periods. As long as the conditions of life remain the same, we have

reason to believe that a modification, which has already been inherited

for many generations, may continue to be inherited for an almost

infinite number of generations. On the other hand we have evidence that

variability, when it has once come into play, does not wholly cease;

for new varieties are still occasionally produced by our most anciently

domesticated productions.

Man does not actually produce variability; he only

unintentionally exposes organic beings to new conditions of life, and

then nature acts on the organisation, and causes variability. But man

can and does select the variations given to him by nature, and thus

accumulate them in any desired manner. He thus adapts animals and

plants for his own benefit or pleasure. He may do this methodically, or

he may do it unconsciously by preserving the individuals most useful to

him at the time, without any thought of altering the breed. It is

certain that he can largely influence the character of a breed by

selecting, in each successive generation, individual differences so

slight as to be quite inappreciable by an uneducated eye. This process

of selection has been the great agency in the production of the most

distinct and useful domestic breeds. That many of the breeds produced

by man have to a large extent the character of natural species, is

shown by the inextricable doubts whether very many of them are

varieties or aboriginal species.

There is no obvious reason why the principles which have acted so

efficiently under domestication should not have acted under nature. In

the preservation of favoured individuals and races, during the

constantly-recurrent Struggle for Existence, we see the most powerful

and ever-acting means of selection. The struggle for existence

inevitably follows from the high geometrical ratio of increase which is

common to all organic beings. This high rate of increase is proved by

calculation, by the effects of a succession of peculiar seasons, and by

the results of naturalisation, as explained in the third chapter. More

individuals are born than can possibly survive. A grain in the balance

will determine which individual shall live and which shall die,—which

variety or species shall increase in number, and which shall decrease,

or finally become extinct. As the individuals

of the same species come in all respects into the closest competition

with each other, the struggle will generally be most severe between

them; it will be almost equally severe between the varieties of the

same species, and next in severity between the species of the same

genus. But the struggle will often be very severe between beings most

remote in the scale of nature. The slightest advantage in one being, at

any age or during any season, over those with which it comes into

competition, or better adaptation in however slight a degree to the

surrounding physical conditions, will turn the balance.

With animals having separated sexes there will in most cases be a

struggle between the males for possession of the females. The most

vigorous individuals, or those which have most successfully struggled

with their conditions of life, will generally leave most progeny. But

success will often depend on having special weapons or means of

defence, or on the charms of the males; and the slightest advantage

will lead to victory.

As geology plainly proclaims that each land has undergone great

physical changes, we might have expected that organic beings would have

varied under nature, in the same way as they generally have varied

under the changed conditions of domestication. And if there be any

variability under nature, it would be an unaccountable fact if natural

selection had not come into play. It has often been asserted, but the

assertion is quite incapable of proof, that the amount of variation

under nature is a strictly limited quantity. Man, though acting on

external characters alone and often capriciously, can produce within a

short period a great result by adding up mere individual differences in

his domestic productions; and every one admits that there are at least

individual differences in species under nature. But, besides such

differences, all naturalists

have admitted the existence of varieties, which they think sufficiently

distinct to be worthy of record in systematic works. No one can draw

any clear distinction between individual differences and slight

varieties; or between more plainly marked varieties and sub-species,

and species. Let it be observed how naturalists differ in the rank

which they assign to the many representative forms in Europe and North

America.

If then we have under nature variability and a powerful agent always

ready to act and select, why should we doubt that variations in any way

useful to beings, under their excessively complex relations of life,

would be preserved, accumulated, and inherited? Why, if man can by

patience select variations most useful to himself, should nature fail

in selecting variations useful, under changing conditions of life, to

her living products? What limit can be put to this power, acting during

long ages and rigidly scrutinising the whole constitution, structure,

and habits of each creature,—favouring the good and rejecting the bad?

I can see no limit to this power, in slowly and beautifully adapting

each form to the most complex relations of life. The theory of natural

selection, even if we looked no further than this, seems to me to be in

itself probable. I have already recapitulated, as fairly as I could,

the opposed difficulties and objections: now let us turn to the special

facts and arguments in favour of the theory.

On the view that species are only strongly marked and permanent

varieties, and that each species first existed as a variety, we can see

why it is that no line of demarcation can be drawn between species,

commonly supposed to have been produced by special acts of creation,

and varieties which are acknowledged to have been produced by secondary

laws. On this same view we can understand how it is that in each region

where many species of a genus have been produced, and where they now

flourish, these same species should present many varieties; for where

the manufactory of species has been active, we might expect, as a

general rule, to find it still in action; and this is the case if

varieties be incipient species. Moreover, the species of the larger

genera, which afford the greater number of varieties or incipient

species, retain to a certain degree the character of varieties; for

they differ from each other by a less amount of difference than do the

species of smaller genera. The closely allied species also of the

larger genera apparently have restricted ranges, and they are clustered

in little groups round other species—in which respects they resemble

varieties. These are strange relations on the view of each species

having been independently created, but are intelligible if all species

first existed as varieties.

As each species tends by its geometrical ratio of reproduction to

increase inordinately in number; and as the modified descendants of

each species will be enabled to increase by so much the more as they

become more diversified in habits and structure, so as to be enabled to

seize on many and widely different places in the economy of nature,

there will be a constant tendency in natural selection to preserve the

most divergent offspring of any one species. Hence during a

long-continued course of modification, the slight differences,

characteristic of varieties of the same species, tend to be augmented

into the greater differences characteristic of species of the same

genus. New and improved varieties will inevitably supplant and

exterminate the older, less improved and intermediate varieties; and

thus species are rendered to a large extent defined and distinct

objects. Dominant species belonging to the larger groups tend to give

birth to new and dominant

forms; so that each large group tends to become still larger, and at

the same time more divergent in character. But as all groups cannot

thus succeed in increasing in size, for the world would not hold them,

the more dominant groups beat the less dominant. This tendency in the

large groups to go on increasing in size and diverging in character,

together with the almost inevitable contingency of much extinction,

explains the arrangement of all the forms of life, in groups

subordinate to groups, all within a few great classes, which we now see

everywhere around us, and which has prevailed throughout all time. This

grand fact of the grouping of all organic beings seems to me utterly

inexplicable on the theory of creation.

As natural selection acts solely by accumulating slight, successive,

favourable variations, it can produce no great or sudden modification;

it can act only by very short and slow steps. Hence the canon of

“Natura non facit saltum,” which every fresh addition to our knowledge

tends to make more strictly correct, is on this theory simply

intelligible. We can plainly see why nature is prodigal in variety,

though niggard in innovation. But why this should be a law of nature if

each species has been independently created, no man can explain.

Many other facts are, as it seems to me, explicable on this theory. How

strange it is that a bird, under the form of woodpecker, should have

been created to prey on insects on the ground; that upland geese, which

never or rarely swim, should have been created with webbed feet; that a

thrush should have been created to dive and feed on sub-aquatic

insects; and that a petrel should have been created with habits and

structure fitting it for the life of an auk or grebe! and so on in

endless other cases. But on the view of each

species constantly trying to increase in number, with natural selection

always ready to adapt the slowly varying descendants of each to any

unoccupied or ill-occupied place in nature, these facts cease to be

strange, or perhaps might even have been anticipated.

As natural selection acts by competition, it adapts the inhabitants of

each country only in relation to the degree of perfection of their

associates; so that we need feel no surprise at the inhabitants of any

one country, although on the ordinary view supposed to have been

specially created and adapted for that country, being beaten and

supplanted by the naturalised productions from another land. Nor ought

we to marvel if all the contrivances in nature be not, as far as we can

judge, absolutely perfect; and if some of them be abhorrent to our

ideas of fitness. We need not marvel at the sting of the bee causing

the bee’s own death; at drones being produced in such vast numbers for

one single act, and being then slaughtered by their sterile sisters; at

the astonishing waste of pollen by our fir-trees; at the instinctive

hatred of the queen bee for her own fertile daughters; at ichneumonidæ

feeding within the live bodies of caterpillars; and at other such

cases. The wonder indeed is, on the theory of natural selection, that

more cases of the want of absolute perfection have not been observed.

The complex and little known laws governing variation are the same, as

far as we can see, with the laws which have governed the production of

so-called specific forms. In both cases physical conditions seem to

have produced but little direct effect; yet when varieties enter any

zone, they occasionally assume some of the characters of the species

proper to that zone. In both varieties and species, use and disuse seem

to have produced some effect; for it is difficult to resist this

conclusion

when we look, for instance, at the logger-headed duck, which has wings

incapable of flight, in nearly the same condition as in the domestic

duck; or when we look at the burrowing tucutucu, which is occasionally

blind, and then at certain moles, which are habitually blind and have

their eyes covered with skin; or when we look at the blind animals

inhabiting the dark caves of America and Europe. In both varieties and

species correlation of growth seems to have played a most important

part, so that when one part has been modified other parts are

necessarily modified. In both varieties and species reversions to

long-lost characters occur. How inexplicable on the theory of creation

is the occasional appearance of stripes on the shoulder and legs of the

several species of the horse-genus and in their hybrids! How simply is

this fact explained if we believe that these species have descended

from a striped progenitor, in the same manner as the several domestic

breeds of pigeon have descended from the blue and barred rock-pigeon!

On the ordinary view of each species having been independently created,

why should the specific characters, or those by which the species of

the same genus differ from each other, be more variable than the

generic characters in which they all agree? Why, for instance, should

the colour of a flower be more likely to vary in any one species of a

genus, if the other species, supposed to have been created

independently, have differently coloured flowers, than if all the

species of the genus have the same coloured flowers? If species are

only well-marked varieties, of which the characters have become in a

high degree permanent, we can understand this fact; for they have

already varied since they branched off from a common progenitor in

certain characters, by which they have come to be specifically distinct

from each other;

and therefore these same characters would be more likely still to be

variable than the generic characters which have been inherited without

change for an enormous period. It is inexplicable on the theory of

creation why a part developed in a very unusual manner in any one

species of a genus, and therefore, as we may naturally infer, of great

importance to the species, should be eminently liable to variation;

but, on my view, this part has undergone, since the several species

branched off from a common progenitor, an unusual amount of variability

and modification, and therefore we might expect this part generally to

be still variable. But a part may be developed in the most unusual

manner, like the wing of a bat, and yet not be more variable than any

other structure, if the part be common to many subordinate forms, that

is, if it has been inherited for a very long period; for in this case

it will have been rendered constant by long-continued natural

selection.

Glancing at instincts, marvellous as some are, they offer no greater

difficulty than does corporeal structure on the theory of the natural

selection of successive, slight, but profitable modifications. We can

thus understand why nature moves by graduated steps in endowing

different animals of the same class with their several instincts. I

have attempted to show how much light the principle of gradation throws

on the admirable architectural powers of the hive-bee. Habit no doubt

sometimes comes into play in modifying instincts; but it certainly is

not indispensable, as we see, in the case of neuter insects, which

leave no progeny to inherit the effects of long-continued habit. On the

view of all the species of the same genus having descended from a

common parent, and having inherited much in common, we can understand

how it is that allied species, when placed under considerably different

conditions of life,

yet should follow nearly the same instincts; why the thrush of South

America, for instance, lines her nest with mud like our British

species. On the view of instincts having been slowly acquired through

natural selection we need not marvel at some instincts being apparently

not perfect and liable to mistakes, and at many instincts causing other

animals to suffer.

If species be only well-marked and permanent varieties, we can at once

see why their crossed offspring should follow the same complex laws in

their degrees and kinds of resemblance to their parents,—in being

absorbed into each other by successive crosses, and in other such

points,—as do the crossed offspring of acknowledged varieties. On the

other hand, these would be strange facts if species have been

independently created, and varieties have been produced by secondary

laws.

If we admit that the geological record is imperfect in an extreme

degree, then such facts as the record gives, support the theory of

descent with modification. New species have come on the stage slowly

and at successive intervals; and the amount of change, after equal

intervals of time, is widely different in different groups. The

extinction of species and of whole groups of species, which has played

so conspicuous a part in the history of the organic world, almost

inevitably follows on the principle of natural selection; for old forms

will be supplanted by new and improved forms. Neither single species

nor groups of species reappear when the chain of ordinary generation

has once been broken. The gradual diffusion of dominant forms, with the

slow modification of their descendants, causes the forms of life, after

long intervals of time, to appear as if they had changed simultaneously

throughout the world. The fact of the fossil remains of each formation

being in some degree intermediate in character between the

fossils in the formations above and below, is simply explained by their

intermediate position in the chain of descent. The grand fact that all

extinct organic beings belong to the same system with recent beings,

falling either into the same or into intermediate groups, follows from

the living and the extinct being the offspring of common parents. As

the groups which have descended from an ancient progenitor have

generally diverged in character, the progenitor with its early

descendants will often be intermediate in character in comparison with

its later descendants; and thus we can see why the more ancient a

fossil is, the oftener it stands in some degree intermediate between

existing and allied groups. Recent forms are generally looked at as

being, in some vague sense, higher than ancient and extinct forms; and

they are in so far higher as the later and more improved forms have

conquered the older and less improved organic beings in the struggle

for life. Lastly, the law of the long endurance of allied forms on the

same continent,—of marsupials in Australia, of edentata in America, and

other such cases,—is intelligible, for within a confined country, the

recent and the extinct will naturally be allied by descent.

Looking to geographical distribution, if we admit that there has been

during the long course of ages much migration from one part of the

world to another, owing to former climatal and geographical changes and

to the many occasional and unknown means of dispersal, then we can

understand, on the theory of descent with modification, most of the

great leading facts in Distribution. We can see why there should be so

striking a parallelism in the distribution of organic beings throughout

space, and in their geological succession throughout time; for in both

cases the beings have been connected by the bond of ordinary

generation, and the means of

modification have been the same. We see the full meaning of the

wonderful fact, which must have struck every traveller, namely, that on

the same continent, under the most diverse conditions, under heat and

cold, on mountain and lowland, on deserts and marshes, most of the

inhabitants within each great class are plainly related; for they will

generally be descendants of the same progenitors and early colonists.

On this same principle of former migration, combined in most cases with

modification, we can understand, by the aid of the Glacial period, the

identity of some few plants, and the close alliance of many others, on

the most distant mountains, under the most different climates; and

likewise the close alliance of some of the inhabitants of the sea in

the northern and southern temperate zones, though separated by the

whole intertropical ocean. Although two areas may present the same

physical conditions of life, we need feel no surprise at their

inhabitants being widely different, if they have been for a long period

completely separated from each other; for as the relation of organism

to organism is the most important of all relations, and as the two

areas will have received colonists from some third source or from each

other, at various periods and in different proportions, the course of

modification in the two areas will inevitably be different.

On this view of migration, with subsequent modification, we can see why

oceanic islands should be inhabited by few species, but of these, that

many should be peculiar. We can clearly see why those animals which

cannot cross wide spaces of ocean, as frogs and terrestrial mammals,

should not inhabit oceanic islands; and why, on the other hand, new and

peculiar species of bats, which can traverse the ocean, should so often

be found on islands far distant from any continent. Such facts

as the presence of peculiar species of bats, and the absence of all

other mammals, on oceanic islands, are utterly inexplicable on the

theory of independent acts of creation.

The existence of closely allied or representative species in any two

areas, implies, on the theory of descent with modification, that the

same parents formerly inhabited both areas; and we almost invariably

find that wherever many closely allied species inhabit two areas, some

identical species common to both still exist. Wherever many closely

allied yet distinct species occur, many doubtful forms and varieties of

the same species likewise occur. It is a rule of high generality that

the inhabitants of each area are related to the inhabitants of the

nearest source whence immigrants might have been derived. We see this

in nearly all the plants and animals of the Galapagos archipelago, of

Juan Fernandez, and of the other American islands being related in the

most striking manner to the plants and animals of the neighbouring

American mainland; and those of the Cape de Verde archipelago and other

African islands to the African mainland. It must be admitted that these

facts receive no explanation on the theory of creation.

The fact, as we have seen, that all past and present organic beings

constitute one grand natural system, with group subordinate to group,

and with extinct groups often falling in between recent groups, is

intelligible on the theory of natural selection with its contingencies

of extinction and divergence of character. On these same principles we

see how it is, that the mutual affinities of the species and genera

within each class are so complex and circuitous. We see why certain

characters are far more serviceable than others for classification;—why

adaptive characters, though of paramount importance to the being, are

of hardly any

importance in classification; why characters derived from rudimentary

parts, though of no service to the being, are often of high

classificatory value; and why embryological characters are the most

valuable of all. The real affinities of all organic beings are due to

inheritance or community of descent. The natural system is a

genealogical arrangement, in which we have to discover the lines of

descent by the most permanent characters, however slight their vital

importance may be.

The framework of bones being the same in the hand of a man, wing of a

bat, fin of the porpoise, and leg of the horse,—the same number of

vertebræ forming the neck of the giraffe and of the elephant,—and

innumerable other such facts, at once explain themselves on the theory

of descent with slow and slight successive modifications. The

similarity of pattern in the wing and leg of a bat, though used for

such different purpose,—in the jaws and legs of a crab,—in the petals,

stamens, and pistils of a flower, is likewise intelligible on the view

of the gradual modification of parts or organs, which were alike in the

early progenitor of each class. On the principle of successive

variations not always supervening at an early age, and being inherited

at a corresponding not early period of life, we can clearly see why the

embryos of mammals, birds, reptiles, and fishes should be so closely

alike, and should be so unlike the adult forms. We may cease marvelling

at the embryo of an air-breathing mammal or bird having branchial slits

and arteries running in loops, like those in a fish which has to

breathe the air dissolved in water, by the aid of well-developed

branchiæ.

Disuse, aided sometimes by natural selection, will often tend to reduce

an organ, when it has become useless by changed habits or under changed

conditions

of life; and we can clearly understand on this view the meaning of

rudimentary organs. But disuse and selection will generally act on each

creature, when it has come to maturity and has to play its full part in

the struggle for existence, and will thus have little power of acting

on an organ during early life; hence the organ will not be much reduced

or rendered rudimentary at this early age. The calf, for instance, has

inherited teeth, which never cut through the gums of the upper jaw,

from an early progenitor having well-developed teeth; and we may

believe, that the teeth in the mature animal were reduced, during

successive generations, by disuse or by the tongue and palate having

been fitted by natural selection to browse without their aid; whereas

in the calf, the teeth have been left untouched by selection or disuse,

and on the principle of inheritance at corresponding ages have been

inherited from a remote period to the present day. On the view of each

organic being and each separate organ having been specially created,

how utterly inexplicable it is that parts, like the teeth in the

embryonic calf or like the shrivelled wings under the soldered

wing-covers of some beetles, should thus so frequently bear the plain

stamp of inutility! Nature may be said to have taken pains to reveal,

by rudimentary organs and by homologous structures, her scheme of

modification, which it seems that we wilfully will not understand.

I have now recapitulated the chief facts and considerations which have

thoroughly convinced me that species have changed, and are still slowly

changing by the preservation and accumulation of successive slight

favourable variations. Why, it may be asked, have all the most eminent

living naturalists and geologists rejected this view of the mutability

of species? It cannot be

asserted that organic beings in a state of nature are subject to no

variation; it cannot be proved that the amount of variation in the

course of long ages is a limited quantity; no clear distinction has

been, or can be, drawn between species and well-marked varieties. It

cannot be maintained that species when intercrossed are invariably

sterile, and varieties invariably fertile; or that sterility is a

special endowment and sign of creation. The belief that species were

immutable productions was almost unavoidable as long as the history of

the world was thought to be of short duration; and now that we have

acquired some idea of the lapse of time, we are too apt to assume,

without proof, that the geological record is so perfect that it would

have afforded us plain evidence of the mutation of species, if they had

undergone mutation.

But the chief cause of our natural unwillingness to admit that one

species has given birth to other and distinct species, is that we are

always slow in admitting any great change of which we do not see the

intermediate steps. The difficulty is the same as that felt by so many

geologists, when Lyell first insisted that long lines of inland cliffs

had been formed, and great valleys excavated, by the slow action of the

coast-waves. The mind cannot possibly grasp the full meaning of the

term of a hundred million years; it cannot add up and perceive the full

effects of many slight variations, accumulated during an almost

infinite number of generations.

Although I am fully convinced of the truth of the views given in this

volume under the form of an abstract, I by no means expect to convince

experienced naturalists whose minds are stocked with a multitude of

facts all viewed, during a long course of years, from a point of view

directly opposite to mine. It is so easy

to hide our ignorance under such expressions as the “plan of creation,”

“unity of design,” etc., and to think that we give an explanation when

we only restate a fact. Any one whose disposition leads him to attach

more weight to unexplained difficulties than to the explanation of a

certain number of facts will certainly reject my theory. A few

naturalists, endowed with much flexibility of mind, and who have

already begun to doubt on the immutability of species, may be

influenced by this volume; but I look with confidence to the future, to

young and rising naturalists, who will be able to view both sides of

the question with impartiality. Whoever is led to believe that species

are mutable will do good service by conscientiously expressing his

conviction; for only thus can the load of prejudice by which this

subject is overwhelmed be removed.

Several eminent naturalists have of late published their belief that a

multitude of reputed species in each genus are not real species; but

that other species are real, that is, have been independently created.

This seems to me a strange conclusion to arrive at. They admit that a

multitude of forms, which till lately they themselves thought were

special creations, and which are still thus looked at by the majority

of naturalists, and which consequently have every external

characteristic feature of true species,—they admit that these have been

produced by variation, but they refuse to extend the same view to other

and very slightly different forms. Nevertheless they do not pretend

that they can define, or even conjecture, which are the created forms

of life, and which are those produced by secondary laws. They admit

variation as a _vera causa_ in one case, they arbitrarily reject it in

another, without assigning any distinction in the two cases. The day

will come when this will be given as a curious illustration of

the blindness of preconceived opinion. These authors seem no more

startled at a miraculous act of creation than at an ordinary birth. But

do they really believe that at innumerable periods in the earth’s

history certain elemental atoms have been commanded suddenly to flash

into living tissues? Do they believe that at each supposed act of

creation one individual or many were produced? Were all the infinitely

numerous kinds of animals and plants created as eggs or seed, or as

full grown? and in the case of mammals, were they created bearing the

false marks of nourishment from the mother’s womb? Although naturalists

very properly demand a full explanation of every difficulty from those

who believe in the mutability of species, on their own side they ignore

the whole subject of the first appearance of species in what they

consider reverent silence.

It may be asked how far I extend the doctrine of the modification of

species. The question is difficult to answer, because the more distinct

the forms are which we may consider, by so much the arguments fall away

in force. But some arguments of the greatest weight extend very far.

All the members of whole classes can be connected together by chains of

affinities, and all can be classified on the same principle, in groups

subordinate to groups. Fossil remains sometimes tend to fill up very

wide intervals between existing orders. Organs in a rudimentary

condition plainly show that an early progenitor had the organ in a

fully developed state; and this in some instances necessarily implies

an enormous amount of modification in the descendants. Throughout whole

classes various structures are formed on the same pattern, and at an

embryonic age the species closely resemble each other. Therefore I

cannot doubt that the theory of descent with modification

embraces all the members of the same class. I believe that animals have

descended from at most only four or five progenitors, and plants from

an equal or lesser number.

Analogy would lead me one step further, namely, to the belief that all

animals and plants have descended from some one prototype. But analogy

may be a deceitful guide. Nevertheless all living things have much in

common, in their chemical composition, their germinal vesicles, their

cellular structure, and their laws of growth and reproduction. We see

this even in so trifling a circumstance as that the same poison often

similarly affects plants and animals; or that the poison secreted by

the gall-fly produces monstrous growths on the wild rose or oak-tree.

Therefore I should infer from analogy that probably all the organic

beings which have ever lived on this earth have descended from some one

primordial form, into which life was first breathed.

When the views entertained in this volume on the origin of species, or

when analogous views are generally admitted, we can dimly foresee that

there will be a considerable revolution in natural history.

Systematists will be able to pursue their labours as at present; but

they will not be incessantly haunted by the shadowy doubt whether this

or that form be in essence a species. This I feel sure, and I speak

after experience, will be no slight relief. The endless disputes

whether or not some fifty species of British brambles are true species

will cease. Systematists will have only to decide (not that this will

be easy) whether any form be sufficiently constant and distinct from

other forms, to be capable of definition; and if definable, whether the

differences be sufficiently important to deserve a specific name. This

latter point will become a far more essential consideration

than it is at present; for differences, however slight, between any two

forms, if not blended by intermediate gradations, are looked at by most

naturalists as sufficient to raise both forms to the rank of species.

Hereafter we shall be compelled to acknowledge that the only

distinction between species and well-marked varieties is, that the

latter are known, or believed, to be connected at the present day by

intermediate gradations, whereas species were formerly thus connected.

Hence, without quite rejecting the consideration of the present

existence of intermediate gradations between any two forms, we shall be

led to weigh more carefully and to value higher the actual amount of

difference between them. It is quite possible that forms now generally

acknowledged to be merely varieties may hereafter be thought worthy of

specific names, as with the primrose and cowslip; and in this case

scientific and common language will come into accordance. In short, we

shall have to treat species in the same manner as those naturalists

treat genera, who admit that genera are merely artificial combinations

made for convenience. This may not be a cheering prospect; but we shall

at least be freed from the vain search for the undiscovered and

undiscoverable essence of the term species.

The other and more general departments of natural history will rise

greatly in interest. The terms used by naturalists of affinity,

relationship, community of type, paternity, morphology, adaptive

characters, rudimentary and aborted organs, etc., will cease to be

metaphorical, and will have a plain signification. When we no longer

look at an organic being as a savage looks at a ship, as at something

wholly beyond his comprehension; when we regard every production of

nature as one which has had a history; when we contemplate every

complex structure

and instinct as the summing up of many contrivances, each useful to the

possessor, nearly in the same way as when we look at any great

mechanical invention as the summing up of the labour, the experience,

the reason, and even the blunders of numerous workmen; when we thus

view each organic being, how far more interesting, I speak from

experience, will the study of natural history become!

A grand and almost untrodden field of inquiry will be opened, on the

causes and laws of variation, on correlation of growth, on the effects

of use and disuse, on the direct action of external conditions, and so

forth. The study of domestic productions will rise immensely in value.

A new variety raised by man will be a far more important and

interesting subject for study than one more species added to the

infinitude of already recorded species. Our classifications will come

to be, as far as they can be so made, genealogies; and will then truly

give what may be called the plan of creation. The rules for classifying

will no doubt become simpler when we have a definite object in view. We

possess no pedigrees or armorial bearings; and we have to discover and

trace the many diverging lines of descent in our natural genealogies,

by characters of any kind which have long been inherited. Rudimentary

organs will speak infallibly with respect to the nature of long-lost

structures. Species and groups of species, which are called aberrant,

and which may fancifully be called living fossils, will aid us in

forming a picture of the ancient forms of life. Embryology will reveal

to us the structure, in some degree obscured, of the prototypes of each

great class.

When we can feel assured that all the individuals of the same species,

and all the closely allied species of most genera, have within a not

very remote period descended

from one parent, and have migrated from some one birthplace; and when

we better know the many means of migration, then, by the light which

geology now throws, and will continue to throw, on former changes of

climate and of the level of the land, we shall surely be enabled to

trace in an admirable manner the former migrations of the inhabitants

of the whole world. Even at present, by comparing the differences of

the inhabitants of the sea on the opposite sides of a continent, and

the nature of the various inhabitants of that continent in relation to

their apparent means of immigration, some light can be thrown on

ancient geography.

The noble science of Geology loses glory from the extreme imperfection

of the record. The crust of the earth with its embedded remains must

not be looked at as a well-filled museum, but as a poor collection made

at hazard and at rare intervals. The accumulation of each great

fossiliferous formation will be recognised as having depended on an

unusual concurrence of circumstances, and the blank intervals between

the successive stages as having been of vast duration. But we shall be

able to gauge with some security the duration of these intervals by a

comparison of the preceding and succeeding organic forms. We must be

cautious in attempting to correlate as strictly contemporaneous two

formations, which include few identical species, by the general

succession of their forms of life. As species are produced and

exterminated by slowly acting and still existing causes, and not by

miraculous acts of creation and by catastrophes; and as the most

important of all causes of organic change is one which is almost

independent of altered and perhaps suddenly altered physical

conditions, namely, the mutual relation of organism to organism,—the

improvement of one being entailing the improvement or the extermination

of

others; it follows, that the amount of organic change in the fossils of

consecutive formations probably serves as a fair measure of the lapse

of actual time. A number of species, however, keeping in a body might

remain for a long period unchanged, whilst within this same period,

several of these species, by migrating into new countries and coming

into competition with foreign associates, might become modified; so

that we must not overrate the accuracy of organic change as a measure

of time. During early periods of the earth’s history, when the forms of

life were probably fewer and simpler, the rate of change was probably

slower; and at the first dawn of life, when very few forms of the

simplest structure existed, the rate of change may have been slow in an

extreme degree. The whole history of the world, as at present known,

although of a length quite incomprehensible by us, will hereafter be

recognised as a mere fragment of time, compared with the ages which

have elapsed since the first creature, the progenitor of innumerable

extinct and living descendants, was created.

In the distant future I see open fields for far more important

researches. Psychology will be based on a new foundation, that of the

necessary acquirement of each mental power and capacity by gradation.

Light will be thrown on the origin of man and his history.

Authors of the highest eminence seem to be fully satisfied with the

view that each species has been independently created. To my mind it

accords better with what we know of the laws impressed on matter by the

Creator, that the production and extinction of the past and present

inhabitants of the world should have been due to secondary causes, like

those determining the birth and death of the individual. When I view

all beings not as special creations, but as the lineal descendants of

some few beings which lived long before the

first bed of the Silurian system was deposited, they seem to me to

become ennobled. Judging from the past, we may safely infer that not

one living species will transmit its unaltered likeness to a distant

futurity. And of the species now living very few will transmit progeny

of any kind to a far distant futurity; for the manner in which all

organic beings are grouped, shows that the greater number of species of

each genus, and all the species of many genera, have left no

descendants, but have become utterly extinct. We can so far take a

prophetic glance into futurity as to foretel that it will be the common

and widely-spread species, belonging to the larger and dominant groups,

which will ultimately prevail and procreate new and dominant species.

As all the living forms of life are the lineal descendants of those

which lived long before the Silurian epoch, we may feel certain that

the ordinary succession by generation has never once been broken, and

that no cataclysm has desolated the whole world. Hence we may look with

some confidence to a secure future of equally inappreciable length. And

as natural selection works solely by and for the good of each being,

all corporeal and mental endowments will tend to progress towards

perfection.

It is interesting to contemplate an entangled bank, clothed with many

plants of many kinds, with birds singing on the bushes, with various

insects flitting about, and with worms crawling through the damp earth,

and to reflect that these elaborately constructed forms, so different

from each other, and dependent on each other in so complex a manner,

have all been produced by laws acting around us. These laws, taken in

the largest sense, being Growth with Reproduction; Inheritance which is

almost implied by reproduction; Variability from the indirect and

direct action of the external conditions

of life, and from use and disuse; a Ratio of Increase so high as to

lead to a Struggle for Life, and as a consequence to Natural Selection,

entailing Divergence of Character and the Extinction of less-improved

forms. Thus, from the war of nature, from famine and death, the most

exalted object which we are capable of conceiving, namely, the

production of the higher animals, directly follows. There is grandeur

in this view of life, with its several powers, having been originally

breathed into a few forms or into one; and that, whilst this planet has

gone cycling on according to the fixed law of gravity, from so simple a

beginning endless forms most beautiful and most wonderful have been,

and are being, evolved.

INDEX.

Aberrant groups, 429.

Abyssinia, plants of, 375.

Acclimatisation, 139.

Affinities:

of extinct species, 329.

of organic beings, 411.

Agassiz:

on Amblyopsis, 139.

on groups of species suddenly appearing, 302, 305.

on embryological succession, 338.

on the glacial period, 366.

on embryological characters, 418.

on the embryos of vertebrata, 439.

on parallelism of embryological development and geological succession,

449.

Algæ of New Zealand, 376.

Alligators, males, fighting, 88.

Amblyopsis, blind fish, 139.

America, North:

productions allied to those of Europe, 371.

boulders and glaciers of, 373.

South, no modern formations on west coast, 290.

Ammonites, sudden extinction of, 321.

Anagallis, sterility of, 247.

Analogy of variations, 159.

Ancylus, 386.

Animals:

not domesticated from being variable, 17.

domestic, descended from several stocks, 19.

acclimatisation of, 141.

of Australia, 116.

with thicker fur in cold climates, 133.

blind, in caves, 137.

extinct, of Australia, 339.

Anomma, 240.

Antarctic islands, ancient flora of, 399.

Antirrhinum, 161.

Ants:

attending aphides, 211.

slave-making instinct, 219.

Ants, neuter, structure of, 236.

Aphides attended by ants, 211.

Aphis, development of, 442.

Apteryx, 182.

Arab horses, 35.

Aralo-Caspian Sea, 339.

Archiac, M. de, on the succession of species, 325.

Artichoke, Jerusalem, 142.

Ascension, plants of, 389.

Asclepias, pollen of, 193.

Asparagus, 359.

Aspicarpa, 417.

Asses, striped, 163.

Ateuchus, 135.

Audubon:

on habits of frigate-bird, 185.

on variation in birds’-nests, 212.

on heron eating seeds, 387.

Australia:

animals of, 116.

dogs of, 215.

extinct animals of, 339.

European plants in, 375.

Azara on flies destroying cattle, 72.

Azores, flora of, 363.

Babington, Mr., on British plants, 48.

Balancement of growth, 147.

Bamboo with hooks, 197.

Barberry, flowers of, 98.

Barrande, M.:

on Silurian colonies, 313.

on the succession of species, 325.

on parallelism of palæozoic formations, 328.

on affinities of ancient species, 330.

Barriers, importance of, 347.

Batrachians on islands, 393.

Bats:

how structure acquired, 180.

distribution of, 394.

Bear, catching water-insects, 184.

Bee:

sting of, 202.

queen, killing rivals, 202.

Bees fertilising flowers, 73.

Bees:

hive, not sucking the red clover, 95.

cell-making instinct, 224.

humble, cells of, 225.

parasitic, 218.

Beetles:

wingless, in Madeira, 135.

with deficient tarsi, 135.

Bentham, Mr.:

on British plants, 48.

on classification, 419.

Berkeley, Mr., on seeds in salt-water, 358.

Bermuda, birds of, 391.

Birds:

acquiring fear, 212.

annually cross the Atlantic, 364.

colour of, on continents, 132.

fossil, in caves of Brazil, 339.

of Madeira, Bermuda, and Galapagos, 390.

song of males, 89.

transporting seeds, 361.

waders, 386.

wingless, 134, 182.

with traces of embryonic teeth, 451.

Bizcacha, 349.

affinities of, 429.

Bladder for swimming in fish, 190.

Blindness of cave animals, 137,

Blyth, Mr.:

on distinctness of Indian cattle, 18.

on striped Hemionus, 163.

on crossed geese, 253.

Boar, shoulder-pad of, 88.

Borrow, Mr., on the Spanish pointer, 35.

Bory St. Vincent on Batrachians, 393.

Bosquet, M., on fossil Chthamalus, 304.

Boulders, erratic, on the Azores, 363.

Branchiæ, 190.

Brent, Mr.:

on house-tumblers, 214.

on hawks killing pigeons, 362.

Brewer, Dr., on American cuckoo, 217.

Britain, mammals of, 395.

Bronn on duration of specific forms, 293.

Brown, Robert, on classification, 414.

Buckman on variation in plants, 10.

Buzareingues on sterility of varieties, 270.

Cabbage, varieties of, crossed, 99.

Calceolaria, 251.

Canary-birds, sterility of hybrids, 252.

Cape de Verde islands, 398.

Cape of Good Hope, plants of, 110, 375.

Carrier-pigeons killed by hawks, 362.

Cassini on flowers of compositæ, 145.

Catasetum, 424.

Cats:

with blue eyes, deaf, 12.

variation in habits of, 91.

curling tail when going to spring, 201.

Cattle:

destroying fir-trees, 71.

destroyed by flies in La Plata, 72.

breeds of, locally extinct, 111.

fertility of Indian and European breeds, 254.

Cave, inhabitants of, blind, 137.

Centres of creation, 352.

Cephalopodæ, development of, 442.

Cervulus, 253.

Cetacea, teeth and hair, 144.

Ceylon, plants of, 375.

Chalk formation, 322.

Characters:

divergence of, 111.

sexual, variable, 156.

adaptive or analogical, 427.

Charlock, 76.

Checks:

to increase, 67.

mutual, 71.

Chickens, instinctive tameness of, 216.

Chthamalinæ, 288.

Chthamalus, cretacean species of, 304.

Circumstances favourable:

to selection of domestic products, 40.

to natural selection, 101.

Cirripedes:

capable of crossing, 101.

carapace aborted, 148.

their ovigerous frena, 192.

fossil, 304.

larvæ of, 440.

Classification, 413.

Clift, Mr., on the succession of types, 339.

Climate:

effects of, in checking increase of beings, 68.

adaptation of, to organisms, 139.

Cobites, intestine of, 190.

Cockroach, 76.

Collections, palæontological, poor, 287.

Colour:

influenced by climate, 132.

in relation to attacks by flies, 198.

Columba livia, parent of domestic pigeons, 23.

Colymbetes, 386.

Compensation of growth, 147.

Compositæ:

outer and inner florets of, 144.

male flowers of, 451.

Conclusion, general, 480.

Conditions, slight changes in, favourable to fertility, 267.

Coot, 185.

Coral:

islands, seeds drifted to, 360.

reefs, indicating movements of earth, 309.

Corn-crake, 185.

Correlation:

of growth in domestic productions, 11.

of growth, 143, 198.

Cowslip, 49.

Creation, single centres of, 352.

Crinum, 250.

Crosses, reciprocal, 258.

Crossing:

of domestic animals, importance in altering breeds, 20.

advantages of, 96.

unfavourable to selection, 102.

Crustacea of New Zealand, 376.

Crustacean, blind, 137.

Cryptocerus, 238.

Ctenomys, blind, 137.

Cuckoo, instinct of, 216.

Currants, grafts of, 262.

Currents of sea, rate of, 359.

Cuvier:

on conditions of existence, 206.

on fossil monkeys, 303.

Cuvier, Fred., on instinct, 208.

Dana, Professor:

on blind cave-animals, 139.

on relations of crustaceans of Japan, 372.

on crustaceans of New Zealand, 376.

De Candolle:

on struggle for existence, 62.

on umbelliferæ, 146.

on general affinities, 430.

De Candolle, Alph.:

on low plants, widely dispersed, 406.

on widely-ranging plants being variable, 53.

on naturalisation, 115.

on winged seeds, 146.

on Alpine species suddenly becoming rare, 175.

on distribution of plants with large seeds, 360.

on vegetation of Australia, 379.

on fresh-water plants, 386.

on insular plants, 389.

Degradation of coast-rocks, 282.

Denudation:

rate of, 285.

of oldest rocks, 308.

Development of ancient forms, 336.

Devonian system, 334.

Dianthus, fertility of crosses, 256.

Dirt on feet of birds, 362.

Dispersal:

means of, 356.

during glacial period, 365.

Distribution:

geographical, 346.

means of, 356.

Disuse, effects of, under nature, 134.

Divergence of character, 111.

Division, physiological, of labour, 115.

Dogs:

hairless, with imperfect teeth, 12.

descended from several wild stocks, 18.

domestic instincts of, 213.

inherited civilisation of, 215.

fertility of breeds together, 254.

of crosses, 268.

proportions of, when young, 444.

Domestication, variation under, 7.

Downing, Mr., on fruit-trees in America, 85.

Downs, North and South, 285.

Dragon-flies, intestines of, 190.

Drift-timber, 360.

Driver-ant, 240.

Drones killed by other bees, 202.

Duck:

domestic, wings of, reduced, 11.

logger-headed, 182.

Duckweed, 385.

Dugong, affinities of, 414.

Dung-beetles with deficient tarsi, 135.

Dyticus, 386.

Earl, Mr. W., on the Malay Archipelago, 395.

Ears:

drooping, in domestic animals, 11.

rudimentary, 454.

Earth, seeds in roots of trees, 361.

Eciton, 238.

Economy of organisation, 147.

Edentata:

teeth and hair, 144.

fossil species of, 339.

Edwards, Milne:

on physiological divisions of labour, 115.

on gradations of structure, 194.

on embryological characters, 418.

Eggs, young birds escaping from, 87.

Electric organs, 192.

Elephant:

rate of increase, 64.

of glacial period, 141.

Embryology, 439.

Existence:

struggle for, 60.

conditions of, 206.

Extinction:

as bearing on natural selection, 109.

of domestic varieties, 111.

317.

Eye:

structure of, 187.

correction for aberration, 202.

Eyes reduced in moles, 137.

Fabre, M., on parasitic sphex, 218.

Falconer, Dr.:

on naturalization of plants in India, 65.

on fossil crocodile, 313.

on elephants and mastodons, 334.

and Cautley on mammals of sub-Himalayan beds, 340.

Falkland Island, wolf of, 393.

Faults, 285.

Faunas, marine, 348.

Fear, instinctive, in birds, 212.

Feet of birds, young molluscs adhering to, 385.

Fertility:

of hybrids, 249.

from slight changes in conditions, 267.

of crossed varieties, 267.

Fir-trees:

destroyed by cattle, 71.

pollen of, 203.

Fish:

flying, 182.

teleostean, sudden appearance of, 305.

eating seeds, 362, 387.

fresh-water, distribution of, 384.

Fishes:

ganoid, now confined to fresh water, 107.

electric organs of, 192.

ganoid, living in fresh water, 321.

of southern hemisphere, 376.

Flight, powers of, how acquired, 182.

Flowers:

structure of, in relation to crossing, 97.

of compositæ and umbelliferæ, 144.

Forbes, E.:

on colours of shells, 132.

on abrupt range of shells in depth, 175.

on poorness of palæontological collections, 287.

on continuous succession of genera, 316.

on continental extensions, 357.

on distribution during glacial period, 366

on parallelism in time and space, 409.

Forests, changes in, in America, 74.

Formation, Devonian, 334.

Formations:

thickness of, in Britain, 284.

intermittent, 290.

Formica rufescens, 219.

Formica sanguinea, 219.

Formica flava, neuter of, 239.

Frena, ovigerous, of cirripedes, 192.

Fresh-water productions, dispersal of, 383.

Fries on species in large genera being closely allied to other species,

57.

Frigate-bird, 185.

Frogs on islands, 393.

Fruit-trees:

gradual improvement of, 37.

in United States, 85.

varieties of, acclimatised in United States, 142.

Fuci, crossed, 258.

Fur, thicker in cold climates, 133.

Furze, 439.

Galapagos Archipelago:

birds of, 390.

productions of, 398, 400.

Galeopithecus, 181.

Game, increase of, checked by vermin, 68.

Gärtner:

on sterility of hybrids, 247, 255.

on reciprocal crosses, 258.

on crossed maize and verbascum, 270.

on comparison of hybrids and mongrels, 272.

Geese:

fertility when crossed, 253.

upland, 185.

Genealogy important in classification, 425.

Geoffrey St. Hilaire:

on balancement, 147.

on homologous organs, 434.

Geoffrey St. Hilaire, Isidore:

on variability of repeated parts, 149.

on correlation in monstrosities, 11.

on correlation, 144.

on variable parts being often monstrous, 155.

Geographical distribution, 346.

Geography, ancient, 487.

Geology:

future progress of, 487.

imperfection of the record, 279.

Giraffe, tail of, 195.

Glacial period, 365.

Gmelin on distribution, 365.

Gnathodon, fossil, 368.

Godwin-Austen, Mr., on the Malay Archipelago, 299.

Goethe on compensation of growth, 147.

Gooseberry, grafts of, 262.

Gould, Dr. A., on land-shells, 397.

Gould, Mr.:

on colours of birds, 132.

on birds of the Galapagos, 398.

on distribution of genera of birds, 404.

Gourds, crossed, 270.

Grafts, capacity of, 261.

Grasses, varieties of, 113.

Gray, Dr. Asa:

on trees of United States, 100.

on naturalised plants in the United States, 115.

on rarity of intermediate varieties, 176.

on Alpine plants, 365.

Gray, Dr. J. E., on striped mule, 165.

Grebe, 185.

Groups, aberrant, 429.

Grouse:

colours of, 84.

red, a doubtful species, 49.

Growth:

compensation of, 147.

correlation of, in domestic products, 11.

correlation of, 143.

Habit:

effect of, under domestication, 11.

effect of, under nature, 134.

diversified, of same species, 183.

Hair and teeth, correlated, 144.

Harcourt, Mr. E. V., on the birds of Madeira, 391.

Hartung, M., on boulders in the Azores, 363.

Hazel-nuts, 359.

Hearne on habits of bears, 184.

Heath, changes in vegetation, 72,

Heer, O., on plants of Madeira, 107.

Helix pomatia, 397.

Helosciadium, 359.

Hemionus, striped, 163.

Herbert, W.:

on struggle for existence, 62.

on sterility of hybrids, 249.

Hermaphrodites crossing, 96.

Heron eating seed, 387.

Heron, Sir R., on peacocks, 89.

Heusinger on white animals not poisoned by certain plants, 12.

Hewitt, Mr., on sterility of first crosses, 264.

Himalaya:

glaciers of, 373.

plants of, 375.

Hippeastrum, 250.

Holly-trees, sexes of, 93.

Hollyhock, varieties of, crossed, 271.

Hooker, Dr., on trees of New Zealand, 100.

Hooker, Dr.:

on acclimatisation of Himalayan trees, 140.

on flowers of umbelliferæ, 145.

on glaciers of Himalaya, 373.

on algæ of New Zealand, 376.

on vegetation at the base of the Himalaya, 378.

on plants of Tierra del Fuego, 374, 378.

on Australian plants, 375, 399.

on relations of flora of South America, 379.

on flora of the Antarctic lands, 381, 399.

on the plants of the Galapagos, 391, 398.

Hooks:

on bamboos, 197.

to seeds on islands, 392.

Horner, Mr., on the antiquity of Egyptians, 18.

Horns, rudimentary, 454.

Horse, fossil, in La Plata, 318.

Horses:

destroyed by flies in La Plata, 72.

striped, 163.

proportions of, when young, 445.

Horticulturists, selection applied by, 32.

Huber on cells of bees, 230.

Huber, P.:

on reason blended with instinct, 208.

on habitual nature of instincts, 208.

on slave making ants, 219.

on Melipona domestica, 225.

Humble-bees, cells of, 225.

Hunter, J., on secondary sexual characters, 150.

Hutton, Captain, on crossed geese, 253.

Huxley, Professor:

on structure of hermaphrodites, 101.

on embryological succession, 338.

on homologous organs, 438.

on the development of aphis, 442.

Hybrids and mongrels compared, 272.

Hybridism, 245.

Hydra, structure of, 190.

Ibla, 148.

Icebergs transporting seeds, 363.

Increase, rate of, 63.

Individuals:

numbers favourable to selection, 102.

many, whether simultaneously created, 356.

Inheritance:

laws of, 12.

at corresponding ages, 14, 86.

Insects:

colour of, fitted for habitations, 84.

sea-side, colours of, 132.

blind, in caves, 138.

luminous, 193.

neuter, 236.

Instinct, 207.

Instincts, domestic, 213.

Intercrossing, advantages of, 96.

Islands, oceanic, 388.

Isolation favourable to selection, 104.

Japan, productions of, 372.

Java, plants of, 375.

Jones, Mr. J. M., on the birds of Bermuda, 391.

Jussieu on classification, 417.

Kentucky, caves of, 137.

Kerguelen-land, flora of, 381, 399.

Kidney-bean, acclimatisation of, 142.

Kidneys of birds, 144.

Kirby on tarsi deficient in beetles, 135.

Knight, Andrew, on cause of variation, 7.

Kölreuter:

on the barberry, 98.

on sterility of hybrids, 247.

on reciprocal crosses, 258.

on crossed varieties of nicotiana, 271.

on crossing male and hermaphrodite flowers, 451.

Lamarck on adaptive characters, 427.

Land-shells:

distribution of, 397.

of Madeira, naturalised, 402.

Languages, classification of, 422.

Lapse, great, of time, 282.

Larvæ, 440.

Laurel, nectar secreted by the leaves, 92.

Laws of variation, 131.

Leech, varieties of, 76.

Leguminosæ, nectar secreted by glands, 92.

Lepidosiren, 107, 330.

Life, struggle for, 60.

Lingula, Silurian, 306.

Linnæus, aphorism of, 413.

Lion:

mane of, 88.

young of, striped, 439.

Lobelia fulgens, 73, 98.

Lobelia, sterility of crosses, 250.

Loess of the Rhine, 384.

Lowness of structure connected with variability, 149.

Lowness, related to wide distribution, 406.

Lubbock, Mr., on the nerves of coccus, 46.

Lucas, Dr. P.:

on inheritance, 12.

on resemblance of child to parent, 275.

Lund and Clausen on fossils of Brazil, 339.

Lyell, Sir C.:

on the struggle for existence, 62.

on modern changes of the earth, 95.

on measure of denudation, 283.

on a carboniferous land-shell, 289.

on fossil whales, 303.

on strata beneath Silurian system, 307.

on the imperfection of the geological record, 310.

on the appearance of species, 312.

on Barrande’s colonies, 313.

on tertiary formations of Europe and North America, 323.

on parallelism of tertiary formations, 328.

on transport of seeds by icebergs, 363.

on great alternations of climate, 382.

on the distribution of fresh-water shells, 385.

on land-shells of Madeira, 402.

Lyell and Dawson on fossilized trees in Nova Scotia, 296.

Macleay on analogical characters, 427.

Madeira:

plants of, 107.

beetles of, wingless, 135.

fossil land-shells of, 339.

birds of, 390.

Magpie tame in Norway, 212.

Maize, crossed, 270.

Malay Archipelago:

compared with Europe, 299.

mammals of, 395.

Malpighiaceæ, 417.

Mammæ, rudimentary, 451.

Mammals:

fossil, in secondary formation, 303.

insular, 393.

Man, origin of races of, 199.

Manatee, rudimentary nails of, 454.

Marsupials:

of Australia, 116.

fossil species of, 339.

Martens, M., experiment on seeds, 360.

Martin, Mr. W. C., on striped mules, 165.

Matteuchi on the electric organs of rays, 193.

Matthiola, reciprocal crosses of, 258.

Means of dispersal, 356.

Melipona domestica, 225.

Metamorphism of oldest rocks, 308.

Mice:

destroying bees, 74.

acclimatisation of, 141.

Migration, bears on first appearance of fossils, 296.

Miller, Professor, on the cells of bees, 226.

Mirabilis, crosses of, 258.

Missel-thrush, 76.

Misseltoe, complex relations of, 3.

Mississippi, rate of deposition at mouth, 284.

Mocking-thrush of the Galapagos, 402.

Modification of species, how far applicable, 483.

Moles, blind, 137.

Mongrels:

fertility and sterility of, 267.

and hybrids compared, 272.

Monkeys, fossil, 303,

Monocanthus, 424.

Mons, Van, on the origin of fruit-trees, 29, 39.

Moquin-Tandon on sea-side plants, 132.

Morphology, 434.

Mozart, musical powers of, 209.

Mud, seeds in, 386.

Mules, striped, 165.

Müller, Dr. F., on Alpine Australian plants, 375.

Murchison, Sir R.:

on the formations of Russia, 289.

on azoic formations, 307.

on extinction, 317.

Mustela vison, 179.

Myanthus, 424.

Myrmecocystus, 238.

Myrmica, eyes of, 240.

Nails, rudimentary, 453.

Natural history:

future progress of, 484.

selection, 80.

system, 413.

Naturalisation:

of forms distinct from the indigenous species, 115.

in New Zealand, 201.

Nautilus, Silurian, 306.

Nectar of plants, 92.

Nectaries, how formed, 92.

Nelumbium luteum, 387.

Nests, variation in, 212.

Neuter insects, 236.

Newman, Mr., on humble-bees, 74.

New Zealand:

productions of, not perfect, 201.

naturalised products of, 337.

fossil birds of, 339.

glacial action in, 373.

crustaceans of, 376.

algæ of, 376.

number of plants of, 389.

flora of, 399.

Nicotiana:

crossed varieties of, 271.

certain species very sterile, 257.

Noble, Mr., on fertility of Rhododendron, 251.

Nodules, phosphatic, in azoic rocks, 307.

Oak, varieties of, 50.

Onites apelles, 135.

Orchis, pollen of, 193.

Organs:

of extreme perfection, 186.

electric, of fishes, 192.

of little importance, 194.

homologous, 434.

rudiments of, 450.

Ornithorhynchus, 107, 416.

Ostrich:

not capable of flight, 134.

habit of laying eggs together, 218.

American, two species of, 349.

Otter, habits of, how acquired, 179.

Ouzel, water, 185.

Owen, Professor:

on birds not flying, 134.

on vegetative repetition, 149.

on variable length of arms in ourang-outang, 150.

on the swim-bladder of fishes, 191.

on electric organs, 192.

on fossil horse of La Plata, 319.

on relations of ruminants and pachyderms, 329.

on fossil birds of New Zealand, 339.

on succession of types, 339.

on affinities of the dugong, 414.

on homologous organs, 435.

on the metamorphosis of cephalopods and spiders, 442.

Pacific Ocean, faunas of, 348.

Paley on no organ formed to give pain, 201.

Pallas on the fertility of the wild stocks of domestic animals, 253.

Paraguay, cattle destroyed by flies, 72.

Parasites, 217.

Partridge, dirt on feet, 362.

Parts:

greatly developed, variable, 150.

degrees of utility of, 201.

Parus major, 183.

Passiflora, 251.

Peaches in United States, 85.

Pear, grafts of, 261.

Pelargonium:

flowers of, 145.

sterility of, 251.

Pelvis of women, 144.

Peloria, 145.

Period, glacial, 365.

Petrels, habits of, 184.

Phasianus, fertility of hybrids, 253.

Pheasant, young, wild, 216.

Philippi on tertiary species in Sicily, 312.

Pictet, Professor:

on groups of species suddenly appearing, 302, 305.

on rate of organic change, 313.

on continuous succession of genera, 316.

on close alliance of fossils in consecutive formations, 335.

on embryological succession, 338.

Pierce, Mr., on varieties of wolves, 91.

Pigeons:

with feathered feet and skin between toes, 12.

breeds described, and origin of, 20.

breeds of, how produced, 39, 42.

tumbler, not being able to get out of egg, 87.

reverting to blue colour, 160.

instinct of tumbling, 214.

carriers, killed by hawks, 362.

young of, 445.

Pistil, rudimentary, 451.

Plants:

poisonous, not affecting certain coloured animals, 12.

selection applied to, 32.

gradual improvement of, 37.

not improved in barbarous countries, 38.

destroyed by insects, 67.

in midst of range, have to struggle with other plants, 77.

nectar of, 92.

fleshy, on sea-shores, 132.

fresh-water, distribution of, 386.

low in scale, widely distributed, 406.

Plumage, laws of change in sexes of birds, 89.

Plums in the United States, 85.

Pointer dog:

origin of, 35.

habits of, 213.

Poison not affecting certain coloured animals, 12.

Poison, similar effect of, on animals and plants, 484.

Pollen of fir-trees, 203,

Poole, Col., on striped hemionus, 163.

Potamogeton, 387.

Prestwich, Mr., on English and French eocene formations, 328.

Primrose, 49.

sterility of, 247.

Primula, varieties of, 49.

Proteolepas, 148.

Proteus, 139.

Psychology, future progress of, 488.

Quagga, striped, 165.

Quince, grafts of, 261.

Rabbit, disposition of young, 215.

Races, domestic, characters of, 16.

Race-horses:

Arab, 35.

English, 356.

Ramond on plants of Pyrenees, 368.

Ramsay, Professor:

on thickness of the British formations, 284.

on faults, 285.

Ratio of increase, 63.

Rats:

supplanting each other, 76.

acclimatisation of, 141.

blind in cave, 137.

Rattle-snake, 201.

Reason and instinct, 208.

Recapitulation, general, 459.

Reciprocity of crosses, 258.

Record, geological, imperfect, 279.

Rengger on flies destroying cattle, 72.

Reproduction, rate of, 63.

Resemblance to parents in mongrels and hybrids, 273.

Reversion:

law of inheritance, 14.

in pigeons to blue colour, 160.

Rhododendron, sterility of, 251.

Richard, Professor, on Aspicarpa, 417.

Richardson, Sir J.:

on structure of squirrels, 180.

on fishes of the southern hemisphere, 376.

Robinia, grafts of, 262.

Rodents, blind, 137.

Rudimentary organs, 450.

Rudiments important for classification, 416.

Sageret on grafts, 262.

Salmons, males fighting, and hooked jaws of, 88.

Salt-water, how far injurious to seeds, 358.

Saurophagus sulphuratus, 183.

Schiödte on blind insects, 138.

Schlegel on snakes, 144.

Sea-water, how far injurious to seeds, 358.

Sebright, Sir J.:

on crossed animals, 20.

on selection of pigeons, 31.

Sedgwick, Professor, on groups of species suddenly appearing, 302.

Seedlings destroyed by insects, 67.

Seeds:

nutriment in, 77.

winged, 146.

power of resisting salt-water, 358.

in crops and intestines of birds, 361.

eaten by fish, 362, 387.

in mud, 386.

hooked, on islands, 392.

Selection:

of domestic products, 29.

principle not of recent origin, 33.

unconscious, 34.

natural, 80.

sexual, 87.

natural, circumstances favourable to, 101.

Sexes, relations of, 87.

Sexual:

characters variable, 156.

selection, 87.

Sheep:

Merino, their selection, 31.

two sub-breeds unintentionally produced, 36.

mountain, varieties of, 76.

Shells:

colours of, 132.

littoral, seldom embedded, 288.

fresh-water, dispersal of, 385.

of Madeira, 391.

land, distribution of, 397.

Silene, fertility of crosses, 257.

Silliman, Professor, on blind rat, 137.

Skulls of young mammals, 197, 437.

Slave-making instinct, 219.

Smith, Col. Hamilton, on striped horses, 164.

Smith, Mr. Fred.:

on slave-making ants, 219.

on neuter ants, 239.

Smith, Mr., of Jordan Hill, on the degradation of coast-rocks, 283.

Snap-dragon, 161.

Somerville, Lord, on selection of sheep, 31.

Sorbus, grafts of, 262.

Spaniel, King Charles’s breed, 35.

Species:

polymorphic, 46.

common, variable, 53.

in large genera variable, 54.

groups of, suddenly appearing, 302, 306.

beneath Silurian formations, 306.

successively appearing, 312.

changing simultaneously throughout the world, 322.

Spencer, Lord, on increase in size of cattle, 35.

Sphex, parasitic, 218.

Spiders, development of, 442.

Spitz-dog crossed with fox, 268.

Sports in plants, 9.

Sprengel, C. C.:

on crossing, 98.

on ray-florets, 145.

Squirrels, gradations in structure, 180.

Staffordshire, heath, changes in, 72.

Stag-beetles, fighting, 88.

Sterility:

from changed conditions of life, 9.

of hybrids, 246.

laws of, 254.

causes of, 263.

from unfavourable conditions, 265.

of certain varieties, 269.

St. Helena, productions of, 389.

St. Hilaire, Aug., on classification, 418.

St. John, Mr., on habits of cats, 91.

Sting of bee, 202.

Stocks, aboriginal, of domestic animals, 18,

Strata, thickness of, in Britain, 284.

Stripes on horses, 163.

Structure, degrees of utility of, 201.

Struggle for existence, 60.

Succession, geological, 312.

Succession of types in same areas, 338.

Swallow, one species supplanting another, 76.

Swim-bladder, 190.

System, natural, 413.

Tail:

of giraffe, 195.

of aquatic animals, 196.

rudimentary, 454.

Tarsi deficient, 135.

Tausch on umbelliferous flowers, 146.

Teeth and hair:

correlated, 144.

embryonic, traces of, in birds, 451.

rudimentary, in embryonic calf, 450, 480.

Tegetmeier, Mr., on cells of bees, 228, 233.

Temminck on distribution aiding classification, 419.

Thouin on grafts, 262.

Thrush:

aquatic species of, 185.

mocking, of the Galapagos, 402.

young of, spotted, 439.

nest of, 243.

Thuret, >M., on crossed fuci, 258.

Thwaites, Mr., on acclimatisation, 140.

Tierra del Fuego:

dogs of, 215.

plants of, 374, 378.

Timber-drift, 360.

Time, lapse of, 282.

Titmouse, 183.

Toads on islands, 393.

Tobacco, crossed varieties of, 271.

Tomes, Mr., on the distribution of bats, 394.

Transitions in varieties rare, 172.

Trees:

on islands belong to peculiar orders, 392.

with separated sexes, 99.

Trifolium pratense, 73, 94.

Trifolium incarnatum, 94.

Trigonia, 321.

Trilobites, 306.

sudden extinction of, 321.

Troglodytes, 243.

Tucutucu, blind, 137.

Tumbler pigeons:

habits of, hereditary, 214.

young of, 446.

Turkey-cock, brush of hair on breast, 90.

Turkey:

naked skin on head, 197.

young, wild, 216.

Turnip and cabbage, analogous variations of, 159.

Type, unity of, 206.

Types, succession of, in same areas, 338.

Udders:

enlarged by use, 11.

rudimentary, 451.

Ulex, young leaves of, 439.

Umbelliferæ, outer and inner florets of, 144.

Unity of type, 206.

Use:

effects of, under domestication, 11.

effects of, in a state of nature, 134.

Utility, how far important in the construction of each part, 199.

Valenciennes on fresh-water fish, 384.

Variability of mongrels and hybrids, 274.

Variation:

under domestication, 7.

caused by reproductive system being affected by conditions of life, 8.

under nature, 44.

laws of, 131.

Variations:

appear at corresponding ages, 14, 86.

analogous in distinct species, 159.

Varieties:

natural, 44.

struggle between, 75.

domestic, extinction of, 111.

transitional, rarity of, 172.

when crossed, fertile, 267.

when crossed, sterile, 269.

classification of, 423.

Verbascum:

sterility of, 251.

varieties of, crossed, 270.

Verneuil, M. de, on the succession of species, 325.

Viola tricolor, 73.

Volcanic islands, denudation of, 284.

Vulture, naked skin on head, 197.

Wading-birds, 386.

Wallace, Mr.:

on origin of species, 2.

on law of geographical distribution, 355.

on the Malay Archipelago, 395.

Wasp, sting of, 202.

Water, fresh, productions of, 383.

Water-hen, 185.

Waterhouse, Mr.:

on Australian marsupials, 116.

on greatly developed parts being variable, 150.

on the cells of bees, 225.

on general affinities, 429.

Water-ouzel, 185.

Watson, Mr. H. C.:

on range of varieties of British plants, 58.

on acclimatisation, 140.

on flora of Azores, 363.

on Alpine plants, 367, 376.

on rarity of intermediate varieties, 176.

Weald, denudation of, 285.

Web of feet in water-birds, 185.

West Indian islands, mammals of, 395.

Westwood:

on species in large genera being closely allied to others, 57.

on the tarsi of Engidæ, 157.

on the antennæ of hymenopterous insects, 416.

Whales, fossil, 303.

Wheat, varieties of, 113.

White Mountains, flora of, 365.

Wings, reduction of size, 134.

Wings:

of insects homologous with branchiæ, 191.

rudimentary, in insects, 451.

Wolf:

crossed with dog, 214.

of Falkland Isles, 393.

Wollaston, Mr.:

on varieties of insects, 48.

on fossil varieties of land-shells in Madeira, 52.

on colours of insects on sea-shore, 132.

on wingless beetles, 135.

on rarity of intermediate varieties, 176.

on insular insects, 389.

on land-shells of Madeira, naturalised, 402.

Wolves, varieties of, 90.

Woodpecker:

habits of, 184.

green colour of, 197.

Woodward, Mr.:

on the duration of specific forms, 293.

on the continuous succession of genera, 316.

on the succession of types, 339.

World, species changing simultaneously throughout, 322.

Wrens, nest of, 243.

Youatt, Mr.:

on selection, 31.

on sub-breeds of sheep, 36.

on rudimentary horns in young cattle, 454.

Zebra, stripes on, 163.